Capture Hood Factor
The Capture Hood Factor is a correction value applied to airflow readings taken by a capture hood to account for the measurement deviation
It is a compensation factor used to convert the measured flow to the true delivered airflow, ensuring:
High accuracy
Reliable balancing results
Full compliance with NEBB/AABC Air System Balancing Procedures
What is Capture Hood Factor?
Why is Capture Hood Factor needed?
The primary reason for applying the factor is that most outlets do not discharge air directly and uniformly into the capture hood. The airflow leaving the diffuser or grille may create resistance, turbulence, and directional throw patterns that prevent the hood from fully capturing the actual delivered air volume.
As a result, the measured airflow may differ from the true airflow.
Therefore, a correction factor must be applied to convert the measured flow into the actual system airflow.
How to Calculate Capture Hood Factor?
The Capture Hood Factor is determined by comparing the airflow measured using the capture hood with the true airflow obtained from a more accurate reference method, typically a duct traverse test, which is considered the standard method for determining actual airflow in the duct.
This method is applicable for each type of standard HVAC outlet, including supply grilles, registers, and diffusers.
Before performing the comparison, the following conditions must be verified:
Ensure there is no air leakage in the system especially at duct joints and at the duct-to-outlet connection
Take both duct traverse and capture hood readings under the same operating conditions to ensure accurate determination of the hood factor
Example
Calculating Capture Hood Factor from Duct Airflow and Hood Reading
In this example, the airflow in the duct is measured using a duct traverse (the most accurate airflow measurement method) and is found to be 420 L/s. Under the same operating conditions, the airflow at the outlet is measured using a capture hood, resulting in a reading of 381 L/s.
To determine the Hood Factor, use the following formula:
Hood Factor = Duct Traverse Airflow ÷ Capture Hood Airflow
Applying the measured values:
Hood Factor = 420 L/s ÷ 381 L/s
Hood Factor = 1.1
The capture hood measurement underestimates the actual airflow due to the airflow pattern at the outlet. By applying the calculated Hood Factor (1.10) to future capture hood readings of the same outlet type and measurement setup, the corrected airflow value will more accurately represent the actual delivered airflow.
